1. Field of the Invention
The present invention relates to an electromechanical transducer such as an ultrasound transducer, and to a method of manufacturing the electromechanical transducer.
2. Description of the Related Art
A capacitive micromachined ultrasound (ultrasonic) transducer (CMUT) is a form of an electromechanical transducer. An example of CMUTs is constructed by electrically connecting a circuit board to a device substrate having multiple devices which include: a substrate having a lower electrode; a membrane, i.e., a vibration film supported by a supporter that is formed on the substrate having a lower electrode; and an upper electrode. Between the substrate having a lower electrode and the membrane, a cavity which is a gap is formed. The CMUT applies a voltage between the lower electrode and the upper electrode to cause the membrane to vibrate and thereby transmit ultrasound waves. The CMUT also receives ultrasound waves, which vibrate the membrane, and detects the ultrasound waves from a change in capacitance between the lower electrode and the upper electrode.
Conventional CMUTs are manufactured by utilizing so-called surface micromachining (surface type) or bulk micromachining (bonding type). A wiring method has been proposed in which multiple membranes and cavities on a silicon substrate constitute one device and the device is connected to a circuit board, with the silicon substrate itself serving as a lower electrode and as through wiring (see Journal of Microelectromechanical Systems, Vol. 17, No. 2, pp. 446-452, April 2008). This method is illustrated in FIG. 4. A device substrate 1007 includes multiple devices 1008, and transmits and receives ultrasound waves with each device 1008 as one unit. Each device 1008 includes an upper electrode 1000, a membrane 1001, cavities 1002, and a lower electrode 1003. Adjacent devices 1008 are electrically isolated (separated) from each other for insulation by a groove 1004 formed between their lower electrodes 1003. Each lower electrode 1003 of the device substrate 1007 is connected via a bump 1005 to a circuit board such as an ASIC substrate. The upper electrodes 1000 of the multiple devices 1008 are connected to an upper electrode lead-out portion 1010, which is connected via upper electrode wiring 1009 and another bump 1005 to the ASIC substrate. Because one lower electrode 1003 is electrically isolated from another in this manner, signals can be taken out on a device basis. The method in Journal of Microelectromechanical Systems, Vol. 17, No. 2, pp. 446-452, April 2008 also gives the CMUT flexibility by filling the groove 1004 with polydimethyl siloxane (PDMS) 1006. The groove 1004 provided for device isolation is sealed with resin to prevent foreign matter from falling into the groove 1004, which is effective in preventing dielectric breakdown between the devices 1008.